Magnetic field effects and excitonic selection rules in monolayer palladium diselenide as a large-gap quantum spin Hall insulator

Abstract

Quantum spin hall (QSH) effect, a class of quantum state, is promising for dissipationless transport with topologically protected helical edge state. Based on first-principles calculations, we theoretically demonstrate the topological physics of $1\mathrm{H}\text{\ensuremath{-}}\mathrm{Pd}{\mathrm{Se}}_{2}$ with a large gap up to 0.24 eV. The band inversion takes place either among the ${p}_{z}$ and ${p}_{x,y}$ orbitals of Se atom or due to spin-orbital coupling under compressive strains. The edge modes exist in the energy gap and can be characterized by an effective edge state Hamiltonian. Based on the derived two-band $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ model, we demonstrate the profound phenomena induced by the applied magnetic field effects and optical transitions for chiral fermions, which lead to complicated excitonic selection rules in this topologically nontrivial two-dimensional material. Our results will pave the way for future theoretical and experimental studies on $\mathrm{Pd}{X}_{2}$ $(X=\mathrm{S},\mathrm{Se},\mathrm{Te})$ monolayers.